This invention relates to a filter plate-and-diaphragm assembly that finds application in a plate-type filter press. The diaphragm is of the type that has, along its outer edge, a peripheral rib. The latter may be inserted into a complemental groove at the sealing edge of the filter plate for anchoring the diaphragm thereto. The filter press comprises a usually great number of such plate-and-diaphragm assemblies arranged in a face-to-face stacked series. Between each filter plate there is disposed a filter cloth, through which the filtrate passes. When the diaphragms are expanded by means of a pressure medium, the residue between the filter cloths is compressed, whereby a filter cake is obtained.
A known pressure diaphragm of the above-outlined type -- as disclosed in German Offenlegungsschrift (Laid Open Application) No. 1,960,821 -- is designed in such manner that the cross-sectional width of the rib provided on the diaphragm and the groove provided in the plate extend over the entire range of the diaphragm zone clamped between two adjoining filter plates. The diaphragm rib thus serves in its entirety as the sole sealing means.
In order to ensure a reliable seal between the diaphragm and the filter plate, a minimum sealing force (compression force) per unit area has to be present.
It is a disadvantage of a diaphragm designed in the above-outlined manner that the minimum sealing pressures have to be substantial in view of the fact that the diaphragm rib fills the entire groove in the filter plate and is of a very considerable width, resulting in a large sealing surface. The requirement for a high sealing pressure, however, necessitates a heavy construction of the filter press components proper. Further, the disadvantage has to be taken into account that the stroke of such filter presses during the compression of the plates is relatively large because the rib thickness, due to the substantial rib width has to be sufficiently great to ensure an effective anchoring of the diaphragm so that the latter does not separate from the groove. Accordingly, in known structures the amount of material used for the diaphragm to obtain a large cross-sectional area of the ribs is also substantial.
In view of the above, it has already been proposed to omit the anchoring ribs at the pressure diaphragms and to glue the diaphragms to the filter plate at the planar marginal diaphragm zones. Such a solution, however, is circumstantial, expensive and further has the disadvantage that during the compression of the components, due to the differences in hardness between the soft diaphragm edge and the hard plate edge, shearing stresses appear which may lead to the destruction of the diaphragm edge, or to a separation of the diaphragm from the filter plate, or to unsealed locations.